1. Field of the Invention
The present invention relates to a satellite communication system and more particularly relates to a satellite communication system for transmitting a message between a mobile station such as an automobile or the like and a stationary station through a communication satellite.
2. Description of the Prior Art
There have previously been developed various types of communication systems wherein a message is transmitted between a cargo transport truck and a command center of a transport company through a communication satellite. As one of these communication systems, a communication system using a so called Geostar (company name) is known. The Geostar system performs a message communication in a manner as shown in FIG. 1 where a message is transmitted between a communication apparatus mounted on a truck 11 and a user center 14 which is a command center of a transport company through a geostationary satellite 12 and an operation center 13. In this case, the transmission of a message between the operation center 13 and the user center 14 is performed through a ground circuit such as a telephone network line or the like, and the message transmission between the operation center 13 and a truck 11 is performed through the geostationary satellite 12. The message repeated or relayed by the geostationary satellite 12 is transmitted with a frequency band of 1.6 GHz, for example. As messages are to be transmitted, a truck 11 transmits information such as departure time, expected arrival time and cargo etc., while the user center 14 transmits informations such as present or current time and road conditions etc.
In case of performing the message transmission as set forth above, a plurality of messages of fixed forms of sentences or standardized sentences to be transmitted are previously prepared and set at a truck 11 so that a crew member of a truck can easily operate the communication apparatus mounted thereon, whereby a crew member is merely required to input only numerals or characters such as time etc., in a part of the standardized sentence to thereby transmit the message of the standardized sentence where the numerals or characters are inserted.
For example, in case of transmitting information as to a departure time of the truck, a crew member selects a previously prepared standardized sentence of "departure time is .sub.-- (hour) .sub.-- (min.)" and inserts numerals indicating the departure time in blanks, whereby information of a code number of this standardized sentence and parts where the numerals representing the departure time is inserted is transmitted as a message to the operation center 13 through the satellite circuit and then transmitted therefrom to the user center 14 through the telephone network circuit 15.
This conventional message transmission using the standardized sentences, however, can only transmit standardized sentences previously prepared in the communication apparatus of the truck 11, and so when it is required to transmit another new message not prepared in the communication apparatus, this requirement could not be satisfied readily.
Namely, in this communication system for a truck, it is sometimes required by a transport company to modify the content of a message, but a standardized sentence is necessarily required to use a code number when transmitting it and so it was difficult for a crew member of a truck to set a standardized sentence and a code number thereof so as to modify the standardized sentence. Thus, the modification operation of a standardized sentence was performed by modifying a standardized sentence set in the communication apparatus mounted on a truck when a truck returned to a truck terminal. However, when a truck is a long distance touring one, a truck scarcely returns to the truck terminal, for example, once every several days or several weeks. Thus, if the modification operation of a standardized sentence set in the communication apparatus mounted on such a long distance touring truck is intended to be performed when the truck returns to the truck terminal, it takes a very long time to complete the modification operation of a standardized sentence of the communication apparatus of each of trucks, so that it is difficult to readily modify a standardized sentence when a requirement of modification of the standardized sentence is raised.